The larval lamprey, Petromyzon marinus, has a retina which is fully differentiated only in the zone surrounding the optic disc. Further development is arrested until metamorphosis and corneal clarification. Only then is physiological and behavioral evidence of visual function noted. The pigment epithelium also shows marked changes in the distribution of its pigment granules over this period. Visual development in the lamprey consists of these and other processes which appear to vary, or even temporarily halt, their progress in time and extent. The goal of this research is to correlate the morphological and electrical events of retinal development during the transformation of the lamprey, Petromyzon marinus, a coldblooded vertebrate. Its eye is easily maintained out of the body for electrophysiology and is of reasonalble size for analysis by light and electron microscopy. By means of these methods, Golgi impregnations and HRP uptake, we will trace the development of the cellular and synaptic organization of the lamprey retina. The formation and turnover of outer segment discs will be studied by radio-labelled aminoacid injection and radioautography. During transformation and the development of the retina, these observations will be corrlelated with the ERG and the appearance of photolabile pigments.